The present invention relates generally to keypad or keyboard data input devices (collectively referred to herein as xe2x80x9ckeyboardsxe2x80x9d) used with general purpose computers and in a wide variety of application specific electronic devices. The invention has particularly useful application in connection with keyboards, e.g., computer keyboards, employing printed membrane switch structures.
To a large extent, the computer industry has settled on using a n(row)xc3x97m(column) matrix of sense lines/drive lines for carrying out keyboard key switch discrimination. Such a matrix provides a means for an integrated circuit (IC) micro-controller device to decode nxc3x97m keys (e.g. 8xc3x9717=136 keyboard keys), with n+m electrical interface pins (e.g. 8+17=25 pins). The micro-controller device sequentially drives a voltage on a plurality of drive lines of the matrix. Keyboard key depression will effect a closure of an associated switch creating an electrical connection between a particular drive line and one of a plurality of sense lines, with the result that the micro-controller device will detect this voltage on the sense line and determine therefrom a corresponding alphanumeric character or function.
Typically, a membrane switch structure is used to form the keyboard key switch matrix. Referring to FIG. 1, a conventional computer keyboard 1 may utilize a conventional membrane switch structure 3, as shown in FIG. 2, which is installed underneath a set of keyboard keys 5. Membrane switch structure 3 generally has three layers 7, 9 and 11. Outer layers 7 and 11 sandwich intermediate layer 9. The three layers are generally made out of a thin insulative sheet of polyester (e.g., Mylar(trademark)) or other insulative material. Outer layers 7 and 11 each have, on their respective opposing inside surfaces 13 and 15, switch circuit patterns (17 and 19, respectively) which may be printed (e.g., silk-screened).
Circuit patterns 17 and 19 are appropriately laid out to provide contact points and lines of conduction for each of keyboard keys 5, within a conventional key switch matrix. The circuit patterns may be printed with suitable conductive inks, e.g., a polymer-based conductive ink having silver and/or carbon particles in suspension. Typically, each keyboard key 21 is coupled to a resilient or spring loaded plunger 23 positioned to make contact with a backside of upper outer layer 7 of membrane switch structure 3. Depression of selected ones of keyboard keys 5 causes a corresponding plunger 23 to exert pressure on upper outer layer 7. The resulting pressure causes a portion of electrical circuit 17 (e.g., a sense line) printed on the inner face of layer 7 to come resiliently into electrical contact with a portion of circuit 19 (e.g., a drive line) printed on the inner face of bottom outer membrane layer 11. This contact occurs through contact apertures 23 (one shown) provided in intermediate layer 9. The electrical contact allows passage of a drive signal on a particular sense line for input to an IC micro-controller device. By recognizing the sense line on which the signal is generated, and the timing of the appearance of the generated signal on the sense line, the IC can discriminate which of keyboard keys 5 has been depressed. The micro-controller device, in turn, provides a digital output signal readable by an associated computer.
FIG. 3 schematically illustrates a conventional arrangement of a computer keyboard switch circuit matrix 25 electrically connected with an IC die package 27 (typically mounted within the keyboard housing). In the conventional arrangement, a total of 17 drive lines 29 extend from IC die package 27 to corresponding drive lines 31 of switch circuit matrix 25. Drive lines 31 are laid-out in intersecting relationship with a total of eight sense lines 33. As previously described, and shown in FIG. 2, the drive lines are typically arranged on a first outer layer of a membrane switch structure, and the sense lines are typically arranged on a second outer layer of a membrane switch structure. This conventional arrangement requires IC die package 27 to accommodate a total of twenty-five (8+17=25) pin connections 35 spaced about the periphery of die package 27, for the signal (drive and sense) lines alone. Additional pin connections (not shown) must also be provided, e.g., for Vcc, Gnd, Osc, HostData and HostClk.
A silicon chip 37 centrally located within die package 27 contains IC logic (including the micro-controller device) for carrying out keyboard key discrimination and related functions. The logic circuitry integrated onto chip 37 is small relative to the available surface area of chip 37 and the high levels of integration attainable with available photolithographic IC fabrication techniques. A much smaller chip (and hence less silicon) could be utilized if it were not for the fact that a certain minimum size of chip 37 is required in order to accommodate pin connections 39 about the periphery of chip 37. Also, the area of chip 37 is small relative to the overall size of die package 27, due to the relatively large space requirements for making the pin connections 35 on the outer periphery of die package 27, as well as pin connections 39 located around the periphery of chip 37. But for the space required to accommodate the large number of required pin connections, the size of die package 27 could be reduced considerably, thus reducing the printed circuit board space required within the keyboard for accommodating die package 27. In addition, a reduction in the number of pin connections would reduce the materials and manufacturing costs associated with making the required pin connections.
The invention provides, in a first aspect, a resistance based keyboard key discrimination system capable of reducing the number of required pin connections to an IC die package. The system includes a plurality of keyboard keys and a switch circuit which may take the form of a current divider circuit or a voltage divider circuit. A plurality of key switches are arranged in the switch circuit to be selectively closed by actuation of corresponding ones of the keyboard keys, and in parallel with each other along a resistive switch line. A voltage source is provided for applying a voltage to the switch line. An A/D converter (A/D) is arranged to sense a voltage in the switch circuit that varies, as a result of resistances in the switch line, in relation to which, if any, of the switches are closed. The A/D outputs a digital value indicative of an actuated keyboard key, for input to logic circuitry of an IC micro-controller.
The switch circuit may include a membrane switch structure with lines of conduction provided (e.g., by silk-screening) on a layer thereof Resistivities of the lines of conduction may be varied by varying one or more of the composition, thickness, width and length of the lines of conduction.
In a second aspect, the invention provides a method of performing keyboard key discrimination in a resistance based keyboard key discrimination system, including a plurality of keyboard keys and a switch circuit. The switch circuit includes a plurality of key switches arranged along a resistive switch line to be selectively closed by actuation of corresponding ones of the keyboard keys. In the method, a voltage is applied to the switch line. A voltage in the switch circuit varies, as a result of resistances in said switch line, in relation to which, if any, of said switches are closed. That voltage is sensed, and a value indicative of an actuated keyboard key is output.
The above and other features and advantages of the present invention will be readily apparent and fully understood from the following detailed description of preferred embodiments, taken in connection with the appended drawings.